diff --git a/src/parsers/smt2/smt2parser.cpp b/src/parsers/smt2/smt2parser.cpp
index c41d8490b..85990c871 100644
--- a/src/parsers/smt2/smt2parser.cpp
+++ b/src/parsers/smt2/smt2parser.cpp
@@ -541,8 +541,6 @@ namespace smt2 {
                     unsigned epos = sexpr_stack().size();
                     SASSERT(epos >= spos);
                     unsigned num  = epos - spos;
-                    if (num == 0)
-                        throw parser_exception("invalid empty s-expression");
                     sexpr * r = sm().mk_composite(num, sexpr_stack().c_ptr() + spos, line, pos);
                     sexpr_stack().shrink(spos);
                     sexpr_stack().push_back(r);
diff --git a/src/sat/sat_drat.cpp b/src/sat/sat_drat.cpp
index 20d7c8f1e..c6e1fc87c 100644
--- a/src/sat/sat_drat.cpp
+++ b/src/sat/sat_drat.cpp
@@ -112,6 +112,11 @@ namespace sat {
             }
         }
 
+        if (n == 3 && c[0] == literal(9357, true) && c[1] == literal(25, true) && c[2] == literal(8691, false)) {
+            SASSERT(false);
+            UNREACHABLE();
+        }
+
         if (!st.is_sat()) {
             for (char ch : m_theory[st.get_th()])
                 buffer[len++] = ch;
diff --git a/src/sat/smt/dt_solver.cpp b/src/sat/smt/dt_solver.cpp
index 9578ff5d3..df9c743fa 100644
--- a/src/sat/smt/dt_solver.cpp
+++ b/src/sat/smt/dt_solver.cpp
@@ -745,8 +745,7 @@ namespace dt {
         else if (is_recognizer(term)) {
             enode* arg = n->get_arg(0);
             theory_var v = mk_var(arg);
-            add_recognizer(v, n);
-            mk_var(n);
+            add_recognizer(v, n);           
         }
         else {
             SASSERT(is_accessor(term));
diff --git a/src/shell/drat_frontend.cpp b/src/shell/drat_frontend.cpp
index 419d9e281..31646009b 100644
--- a/src/shell/drat_frontend.cpp
+++ b/src/shell/drat_frontend.cpp
@@ -188,6 +188,15 @@ public:
                     result = bvu.mk_numeral(val, n);
                     return;
                 }
+                if (name == "is" && sz == 3) {
+                    name = sexpr->get_child(2)->get_child(0)->get_symbol();
+                    f = ctx.find_func_decl(name, params.size(), params.c_ptr(), args.size(), sorts.c_ptr(), rng.get());
+                    if (!f)
+                        goto bail;
+                    datatype_util dtu(m);
+                    result = dtu.mk_is(f, args[0]);
+                    return;
+                }
                 for (unsigned i = 2; i < sz; ++i) {
                     auto* child = sexpr->get_child(i);
                     if (child->is_numeral() && child->get_numeral().is_unsigned())
diff --git a/src/util/debug.h b/src/util/debug.h
index 170049bdb..9ad55a7c2 100644
--- a/src/util/debug.h
+++ b/src/util/debug.h
@@ -64,9 +64,9 @@ bool is_debug_enabled(const char * tag);
 #define XASSERT(COND, EXTRA_CODE) DEBUG_CODE(if (assertions_enabled() && !(COND)) { notify_assertion_violation(__FILE__, __LINE__, #COND); { EXTRA_CODE } INVOKE_DEBUGGER(); })
 
 #ifdef Z3DEBUG
-# define UNREACHABLE() DEBUG_CODE(notify_assertion_violation(__FILE__, __LINE__, "UNREACHABLE CODE WAS REACHED."); INVOKE_DEBUGGER();)
+# define UNREACHABLE() DEBUG_CODE(notify_assertion_violation(__FILE__, __LINE__, "UNEXPECTED CODE WAS REACHED."); INVOKE_DEBUGGER();)
 #else
-# define UNREACHABLE() { notify_assertion_violation(__FILE__, __LINE__, "UNREACHABLE CODE WAS REACHED."); exit(ERR_UNREACHABLE); } ((void) 0)
+# define UNREACHABLE() { notify_assertion_violation(__FILE__, __LINE__, "UNEXPECTED CODE WAS REACHED."); exit(ERR_UNREACHABLE); } ((void) 0)
 #endif
 
 #ifdef Z3DEBUG
diff --git a/src/util/sexpr.cpp b/src/util/sexpr.cpp
index 93acf7a45..53de22d79 100644
--- a/src/util/sexpr.cpp
+++ b/src/util/sexpr.cpp
@@ -194,6 +194,8 @@ void sexpr::display(std::ostream & out) const {
         sexpr_composite const * n = todo.back().first;
         unsigned & idx = todo.back().second;
         unsigned num = n->get_num_children();
+        if (num == 0)
+            out << "(";
         while (idx < num) {
             sexpr const * child = n->get_child(idx);
             if (idx == 0)